kinematic and kinetic comparison of running in standard
TRANSCRIPT
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Kinematic and kinetic comparison of running in standard and minimalist shoes
Richard W. Willy, PhD, PT, OCS1
Irene S. Davis, PhD, PT, FAPTA, FACSM, FASB2
1Department of Physical Therapy, East Carolina University, Greenville, NC, USA. No declared
conflict of interest. Dr. Willy has no disclosures of professional relationships with companies or
manufacturers who may/will benefit from the results of this present study.
2Spaulding National Running Center, Department of Physical Medicine and Rehabilitation,
Harvard Medical School, Cambridge, MA, USA, No declared conflict of interest. Dr. Davis has
no disclosures of professional relationships with companies or manufacturers who may/will
benefit from the results of this present study.
Disclosure of funding: Drayer Physical Therapy Institute, DOD W911NF-05-1-0097, NIH 1 S10
RR022396
The protocol for this study was approved by The University of Delaware Human Subjects
Compliance Committee.
Corresponding Author:
Richard Willy, PT, PhD
Department of Physical Therapy
College of Allied Health Sciences
East Carolina University
Greenville, NC 27834 USA
Mail stop: 668 Allied Health
email: [email protected]
Running Title: Running in standard versus minimalist shoes
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ABSTRACT
Purpose: The purpose of this study was to determine if running in a minimalist shoe results in a
reduction in ground reaction forces and alters kinematics over standard shoe running. The
secondary purpose of this study was to determine if within-session accommodation to a novel
minimalist shoe occurs.
Methods: Subjects were 14 male, rearfoot striking runners who had never run in a minimalist
shoe. Subjects were tested while running 3.35 m/s for 10 minutes on an instrumented treadmill in
a minimalist and a standard shoe as 3-D lower extremity kinematics and kinetics were evaluated.
Data were collected at minute 1 and then again after 10 minutes of running in both shoe
conditions to evaluate accommodation to the shoe conditions.
Results: Shoe x time interactions were not found for any of the variables of interest. Minimalist
shoe running resulted in no changes in step length (p=0.967) nor step rate (p=0.230). At
footstrike, greater knee flexion (p=0.001) and greater dorsiflexion angle (p=0.025) were noted in
the minimalist shoe. Vertical impact peak (p=0.017) and average vertical loading rate (p<0.000)
were greater during minimalist shoe running. There were main effects of time as dorsiflexion
angle decreased (p=0.035), foot inclination at footstrike decreased (p=0.048) and knee flexion at
footstrike increased (p=0.002), yet the vertical impact peak (p=0.002) and average vertical
loading rate (p<0.000) increased.
Conclusions: Running in a minimalist shoe appears to, at least in the short-term, increase
loading of the lower extremity over standard shoe running. The accommodation period resulted
in less favorable landing mechanics in both shoes. These findings bring into question whether
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minimal shoes will provide enough feedback to induce an alteration that is similar to barefoot
running.
Key words: Ground reaction forces, injury prevention, biomechanics, loading rate
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INTRODUCTION
Paragraph Number 1: Common running injuries, such as patellofemoral pain, plantar fasciitis
and tibial stress fractures have been associated with high impact forces during running.(9, 16, 19,
20, 25) In particular, a high vertical impact peak in the vertical ground reaction force curve and a
high rate of rise to this vertical impact peak (vertical loading rate) have been associated with
these injuries.(16, 19, 20, 25) The vertical impact force transient is typically seen in runners who
strike the ground first with their heel and signifies the end of the passive phase of loading.(1, 6)
Thus, it appears that mitigating the forces associated with the passive impact transient should be
the focus for reducing the risk of many running-related injuries.
Paragraph Number 2: Various interventions have been suggested to reduce impact forces
associated with running. These include the use of cushioned shoes and insoles(4, 5), as well as
conscious alterations in running technique, also known as gait retraining.(7, 8, 12, 14, 17, 22, 23)
It has been shown that adopting a forefoot or midfoot strike pattern reduces or eliminates the
vertical impact peak and reduces the vertical loading rate.(7, 15, 23) Reduction of impact loading
may also be accomplished by increasing step rate and reducing step length.(14) A reduction in
stride length is often accomplished a strategy of increased knee flexion at foot strike and reduced
vertical oscillation of the runner’s center of mass.(14)
Paragraph Number 3: Recently, barefoot running has been suggested as a means to reduce
impact forces.(1) Habitual barefoot runners tend to strike the ground with a less dorsiflexed foot,
often utilizing a mid- or a forefoot strike pattern.(1, 10, 12, 15) It is reasoned that barefoot
runners adopt a less dorsiflexed strike pattern because heelstriking while barefoot is painful.(1,
10, 15) In fact, heelstriking while barefoot running results in a very high vertical impact peak
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and resultant vertical loading rate due to the collision forces between the dense calcaneus and the
ground.(15) For many runners, barefoot running may be impractical due to unsafe running
surfaces and potential performance limitations.(13). Thus, many runners opt for an intermediate
option, a minimalist shoe. Minimalist shoe running may have an even lower metabolic cost
compared with standard shoe running.(18) Nevertheless, there is currently a wide variation of
shoes that are advertised as “minimal.” All of these shoes are commercially marketed as an
alternative to barefoot running.
Paragraph Number 5: Only two studies, to date, have examined the effects of minimalist
footwear on kinematic and kinetic parameters of running. Squadrone et al. (2010) examined
habitual barefoot runners during three conditions: barefoot, minimalist and standard shoe
running.(21) The minimalist shoe used was a 5-toed shoe with a flexible upper, no arch support,
a zero drop, no midsole and a 3 mm outersole and the standard shoe was described as a neutral
shoe. Using a 2-D kinematic assessment and an instrumented treadmill, barefoot and minimalist
shoe running both resulted in a reduction in sagittal plane ankle and knee angle just prior to
footstrike compared with standard shoe running. In addition, they reported a reduction in strike
index, stride length, and an increase in step rate. also resulted.(21). Finally, the vertical impact
peak was found to be reduced in both the barefoot and minimal footwear conditions. However,
these authors did not assess vertical loading rate in this study. Further, habitual barefoot runners
were the participants. Therefore, their results may be due to long term changes in running
mechanics due to barefoot running than any immediate effect of footwear. More recently,
Bonacci et al. examined the running mechanics of highly trained runners while running in
standard (each runner’s regular training shoe), lightweight (Nike LunaRacer2), minimalist (Nike
Free 3.0), and barefoot conditions.(3) The lightweight and minimalist shoe were similar in the
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amount of cushioning they had. As a result, these two shoes resulted in very similar mechanics
to each other. Runners did exhibit a reduced stride length and increased cadence in these two
shoes compared to the standard shoe, but most all other variables were similar to standard shoe
condition. Neither of these more minimal shoes resulted in mechanics similar to barefoot
running. This study underscores the idea that cushioned shoes, even when there is less
cushioning than standard shoes, still encourage a heelstrike pattern. The only condition where a
plantarflexed position at footstrike was noted was the barefoot one. Bonacci et al. did not report
on impact-related forces between the shoe conditions. Understanding the influence of footwear
on vertical impact peaks and loadrates is critical, as these forces have been related to running
injuries.
Paragraph Number 6: Therefore, the purpose of this investigation was to compare running
mechanics between standard neutral cushioned shoes and minimalist shoes in novice minimalist
shoe runners. We also sought to determine if runners are able to accommodate to the minimal
cushioning in minimalist shoes within a single running session. We hypothesized that runners
would increase step rate while decreasing stance time in the minimalist shoes, resulting in a
reduction in impact forces when compared with a standard cushioned shoe. An associated
decrease in ankle dorsiflexion and increase in knee flexion at footstrike would also be noted in
the minimalist shoe condition. We also hypothesized that runners would increase step rate,
decrease impact forces and alter their running kinematics to an even greater extent over the
course of the run as runners became accommodated to the minimalist shoe.
METHODS
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Paragraph Number 7: The data collection protocol and informed consent document were
approved by the University of Delaware Human Subjects Research Board. In order to participate,
both written and verbal informed consent was obtained from each volunteer. An a-priori power
analysis was conducted using data from pilot work for this study. Using the variable with the
highest standard deviation, ankle dorsiflexion at footstrike, it was revealed that 12 subjects
(effect size=0.82, α= 0.05, β=0.20) were required to adequately power this study. To be
conservative, we recruited 14 male runners (31.9 km/week ±10.5, 24.8 years of age ±3.2) for this
study. In order to qualify, each runner was required to be a habitual shod heelstriker, running at
least 10 miles/week, between 18-35 years of age, free of injury over the past 6 months. Habitual
strike pattern was self-reported. Each runner was required to be comfortable with treadmill
running, defined as a score of at least “8” on a visual analog scale (“0” and “10” corresponding
to completely uncomfortable versus completely comfortable, respectively). They also had to be
comfortable running at 3.3 m/s. Importantly, each runner was required to be a “novel”
minimalist shoe wearer, defined as never having previously run in minimalist shoes. Shoe history
was self-reported. We operationally defined a “minimalist shoe” as a racing flat or advertised
minimalist shoe. We chose to examine novel minimalist shoe runners in order to capture the true
accommodation that may occur during the initial exposure to minimalist shoe running.
Paragraph Number 8: Shoe order was counterbalanced among subjects to avoid a fatigue and
learned effect. A Nike Pegasus (Nike, Beaverton, Oregon) served as the standard cushioned shoe
and the Nike Free 3.0 served as the minimalist shoe. The Nike Pegasus and Free shoes have heel
insole heights of 36.3 mm and 17.6 mm, respectively, as per caliper measurement of size 47
EUR. A MTS QTest 10 Elite load frame (Cary, NC) with a 10 KN load cell was used to measure
the stiffness of the heel insole. One shoe from each group was tested once. A compressive
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preload of 100 N was applied, then the load was increased to 1000 N with a load rate of 400 N/s.
Stiffness values of 64.5 N/mm and 88.2 N/mm were determined for the standard and minimalist
shoes, respectively. Thus, the standard shoe provided 31% greater cushioning than the
minimalist shoe.
Paragraph Number 9: Thirty-five retroreflective markers were attached to the dominant lower
extremity to analyze running kinematics (VICON, Oxford, UK). Limb dominance was
operationally defined as the leg used to kick a soccer ball. Marker bases were firmly attached to
bony landmarks to establish the coordinate systems of the pelvis, thigh, shank, and foot. Marker
placement on shoes has previously been shown to overestimate calcaneal movement during
running. Therefore, shoe windows were utilized to facilitate marker placement directly on the
calcaneus (Figure 1).
Paragraph Number 10: Efforts were made to minimize any potential offsets that could be
introduced to the subject models as a result of the different footwear conditions and variability in
marker placement. Separate standing calibration trials were taken for each shoe condition
utilizing identical anatomical marker placements. This was accomplished by using markers that
could easily be separated from their bases, thus leaving the bases in place during the running
trials. Once the first run condition was completed, the markers were reattached to their
previously mounted bases for the subsequent standing calibration trial. Finally, all tracking
markers, including markers mounted directly on the calcaneus, remained in place and unchanged
between shoe running conditions.
Paragraph Number 11: For the collection of running trials, subjects ran on an instrumented
treadmill (AMTI, Watertown, MA) at 3.35 meters/sec (8 min/mile pace). To fully capture any
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accommodation to shoe conditions, no warm-up period was allowed. This was accomplished by
accelerating the treadmill from a full stop to the test speed using an acceleration rate of 0.2
meters/sec2
as the subject kept pace. This acceleration rate is modest and was deemed to be a
comfortable and attainable acceleration rate by each participant. Data of 5 consecutive strides
were analyzed in the first minute of running as soon as the test speed was reached and after 10
minutes of running. Kinematic and kinetic data were collected at 200- and 1000-Hz, respectively.
Stance was determined using a threshold of 30 N of the vertical ground reaction force. Once
data were collected after the 10th
minute, the treadmill was stopped and the shoe condition was
changed. Care was exercised to maintain the calcaneal markers in their original positions as
shoes were fully unlaced and removed followed by replacing them with the second shoe
condition. Anatomical markers were reattached to their previously mounted bases. The second
testing session was then commenced and data were recorded in exactly the same manner as
described for the first shoe condition.
Paragraph Number 12: Data were then processed using Visual 3D (CMotion, Bethesda, MD).
To eliminate any offset introduced into the ankle kinematic data due to the differences in heel
height among the two shoe conditions, a virtual foot was created. This was done by subtracting
the vertical height of the foot and malleoli anatomical markers in the lab coordinate system to
construct the virtual segment. As such, the virtual foot had an inclination (dorsiflexion) angle of
0 degrees during the standing calibration, when referenced to the lab coordinate system.
Subsequent foot kinematic calculations were based on the virtual foot referenced to the lab
coordinate system and ankle kinematic calculations were based on the virtual foot referenced to
the shank segment. Using this virtual foot, a positive angle of the foot inclination angle at
footstrike would correspond to a dorsiflexed foot segment at foot strike. An 8- and 40-Hz, low-
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pass, 4th
order, zero-lag Butterworth filters were used to filter the kinematic and kinetic data,
respectively. Customized software (LabVIEW 8.0, National Instruments, Austin TX) was used to
calculate the following discreet variables: vertical impact peak, average vertical loading rate,
dorsiflexion at footstrike, foot inclination at footstrike, knee angle at footstrike, stance length and
step rate. The methodology for calculation of average vertical loading rate was done over the
middle 60% of the vertical ground reaction force curve from foot strike to the vertical impact
peak (Figure 2). (16)
Paragraph Number 13: A series of two-way, repeated analyses of variance (ANOVA) (shoe(2) x
time(2)) was utilized to analyze the data (α≤0.05). When significant differences were found for
main effects or the interaction, post hoc comparisons using paired t-tests (α<0.05) were
conducted between levels. Mauchly’s test of sphericity was utilized to determine if the data met
the assumptions of the ANOVA.
RESULTS
Paragraph Number 14: There were no significant shoe X time interactions, and therefore only
main effects were assessed. In terms of main effect of shoe-type, no differences were found for
step length, step rate, foot inclination angle at foot strike (p=0.967, F=0.002, p=0.230, F=1.586,
and p=0.332, F=1.014, respectively) between shoes. However, runners struck the ground with a
more dorsiflexed foot (p=0.025, F=6.379), in more knee flexion (p=0.001, F=13.000), and with
higher vertical impact peak (p=0.017, F=14.902) and higher average vertical loading rate
(p<0.000, F=23.400) (Table 1) while running in the minimalist shoe. These results did not agree
with our hypotheses that runners would land in a less foot inclination and with a less dorsiflexed
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ankle, experience lower impact forces, and reduce temporospatial measures in the minimalist
shoe when compared with the standard running shoe.
Paragraph Number 15: There were no main effects of time for step length or step rate either
(p=0.088, F=3.393 and p=0.616, F=0.265, respectively). Significant main effects for time for all
kinematic and kinetic variables were found. Runners in both shoe conditions demonstrated
reduced foot inclination at footstrike (p=0.048, F=4.763), reduced dorsiflexion (p=0.035,
F=5.543) and increased knee flexion at foot strike (p=0.002, F=14.112), yet higher vertical
impact peak (p=0.002, F=14.902) and average vertical loading rate (p<0.000, F=91.884) after 10
minutes of running.
DISCUSSION
Paragraph Number 16: We sought to determine if running in a cushioned minimalist shoe results
in reduced variables associated with impacts when compared with running in a standard shoe. In
addition, we sought to determine if accommodation to running in the minimalist shoe occurs
over the course of a 10-minute run. In contrast to our hypotheses, we found that running in the
minimalist shoe failed to result in changes in temporospatial parameters, increased average
vertical loading rates and vertical impact peaks when compared with running in a standard
running shoe. As hypothesized, running in the minimalist shoe resulted in a slightly more flexed
knee at footstrike. The runners accommodated to the minimalist shoes in the same manner as to
the standard shoes.
Paragraph Number17: The minimalist shoe resulted in a more dorsiflexed ankle and more knee
flexion at footstrike compared with standard shoe running. This increased knee flexion may have
been a strategy to reduce the impact forces associated with a more dorsiflexed footstrike.(14)
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However, this increase in knee flexion was small and apparently insufficient to alter the
temporospatial measures nor reduce impact forces during minimalist shoe running.
Paragraph Number 18: It is not clear why dorsiflexion would be increased in the minimal shoe,
yet foot inclination was unchanged. Foot inclination was defined as the virtual foot referenced to
the running surface whereas ankle dorsiflexion was the foot referenced to the shank segment.
Therefore, foot inclination is likely a better indication of foot strike pattern than ankle
dorsiflexion angle. Previous work suggests that foot inclination at foot strike is strongly
correlated with foot strike patterns.(2) Therefore, we feel that the large (>10°) positive foot
inclination angle at foot strike in both shoes indicates a defined heelstrike in the present study.
As a large positive foot inclination angle at foot strike was coupled with a stiffer, less compliant
minimalist shoe, there was less cushioning between the dense calcaneus and the running surface
compared with the standard shoe. Thus, it is not surprisingly then that the minimalist shoe was
associated with greater impact loading. Landing on the heel reduces the ability of the ankle to
assist in attenuating the loads of impact. High impact loading has been associated with a number
of running related injuries.(9, 16, 19, 20, 25) In fact, the average vertical loading rates that we
found during minimalist shoe running exceeded the values reported in runners with a history of
tibial stress fractures by Milner et al. (78.97 bw/sec, ± 24.96).(16) This is amplified by the fact
that the running speed was higher in the Milner study than in the present one (3.7 vs 3.3 m/s) as
higher ground reaction forces would be expected at the faster speed. Further justification of this
comparison is that there are no differences in impact forces between overground and treadmill
running.(24)
Paragraph Number 19: The greater ankle dorsiflexion seen in the minimal shoe was in contrast
to the findings of both Squadrone and Bonacci. Squadrone found that habitually barefoot
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runners in the Vibram five finger shoe landed in plantarflexion compared to dorsiflexion in the
standard running shoes.(21) The plantarflexion found in the Vibram five finger shoe was also
similar to that of their barefoot condition. However, these runners were habitual barefoot
runners and the minimal shoe had no cushioning at all. Bonacci found no difference between the
Nike Free 3.0 and the runner’s standard shoes, which agrees with our findings.(3) The Bonacci
study used a similar shoe to the present study, and runners were given 10 days to accommodate
to the shoes. The Bonacci study used a similar shoe to the present study, but runners were given
10 days to accommodate to the shoes which may explain some of the difference. The virtual foot
may also at least partially be the source of this discrepancy in dorsiflexion angle between the
present work and that of Squadrone and Bonacci. While we accounted for foot angle differences
between shoe types, we did not account for differences in shank orientation that may have
resulted from the greater heel elevation in the neutral shoe.
Paragraph Number 20: Our results also suggest that a 10-minute accommodation period does
not result in more favorable loading mechanics during running in the minimalist shoe. In fact,
any changes in mechanics between minute 1 and minute 10 were nearly identical between the
two shoe types. Rather than decreasing their impact loading as the runners became
accommodated, an increase was found between the two time points in both shoes. While
impacts have been noted to increase with fatigue,(11) 10 minutes of running should not have led
to fatigue in this group of runners. This may be explained by the fact that the Pegasus may have
been an equally novel shoe for the runners as the minimalist shoe. An alternative explanation is
that minimizing impact forces is not a criterion of the neuromuscular system during the initial
minutes of running.
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Paragraph Number 21: When taking the Squadrone, Bonacci and current studies together, the
following points can be made. In order to see a change in footstrike pattern that simulates
barefoot running, the shoes may need to be as minimal as possible. The shoe in the Squadrone
study was simply a thin rubber sole with a flexible upper. It was the only shoe condition of all
three studies that resulted in similar mechanics as a barefoot condition. The Squadrone study
used habitual barefoot runners who may also do some of their running in minimal shoes.(21)
Although the standard shoe condition was the one that was most unfamiliar, runners in the
Squadrone study immediately increased their dorsiflexion at footstrike and increased their
vertical impact peak. In both the Bonacci and present study, the minimal shoe used was one that
still had significant cushioning and allowed for a comfortable heelstrike.(3) In both cases,
runners remained as rearfoot strikers in the minimal shoe condition, landing on their heel with
greater impact.
Paragraph Number 22: Many runners use these transitional shoes as a way to adopt a barefoot-
mimicking running technique, involving minimal plantarflexion and a mild forefoot strike
pattern.(15) However, these results suggest that runners should be careful when transitioning to
cushioned minimal shoes. While there may be a belief that one will automatically adopt a
forefoot or midfoot strike pattern by wearing minimal shoes, both the Bonacci and the present
study suggests otherwise. When using one of the vast array of minimal shoes with cushioning,
additional gait retraining may be needed in order to train one to land with a midfoot or mild
forefoot strike pattern. Based upon previous studies, landing patterns can be altered (7, 8, 14)
and with proper feedback and reinforcement, maintained up to 3 months beyond the training
period.(7, 8) Whether this alteration is maintained in the long term when using cushioned
minimal shoes is yet to be determined.
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CONCLUSION
Paragraph Number 23: These results suggest that the minimally cushioned footwear used in the
study did not induce result in a transition to a non-rearfoot strike pattern. As runners actually
increased their impact loading, these shoes may increase the risk of injury, especially during this
early phase of accommodation.
ACKNOWLEDGEMENTS
Paragraph Number 24: The authors would like to acknowledge our funding sources: Drayer
Physical Therapy Institute, DOD W911NF-05-1-0097, NIH 1 S10 RR022396. The authors
would also like to thank the subjects for their participation in this study. The results of the
present study do not constitute endorsement by the American College of Sports Medicine
(ACSM). Lindsay Buchenic assisted with processing of the running data. William Zaylor
performed the materials testing on shoe stiffness qualities.
CONFLICT OF INTEREST
Paragraph Number 27: There are no conflicts of interest among any of the authors of this
manuscript.
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Table Captions
Table 1: Variables of interest. Mean (standard deviation). A more negative knee angle
corresponds with a more flexed knee.
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Figure Captions
Figure 1: Marker set utilized for collection of motion data. Note shoe windows enabling
placement of marker directly on calcaneus.
Figure 2: Ensemble curves of the vertical ground reaction forces (GRF) during Minute 1 of
treadmill running in the two shoe conditions. VIP=vertical impact peak, AVLR= average vertical
loading rate. * indicates p<0.05
